ABSTRACT Alcohol and marijuana are the most commonly used substances among young adults (YAs) and their use is often intertwined. With recent changes in legalization of marijuana in several states, it is critical to understand the impact marijuana use may have on alcohol use. In states like Washington where both alcohol and recreational marijuana use are legal for adults (aged 21+), it is important to examine how these two substances are being used and interacting among YAs. Increases in marijuana use may be associated with decreases in alcohol use if marijuana acts as a substitute for alcohol, or it may lead to greater alcohol use if there are complementary effects such that both marijuana and alcohol use increase over time. To date, most study designs have not provided data to allow examination, at the individual level, of whether there are prospective month-to-month substitution or complementary effects over time and there are no studies to date that have examined these effects in the context of legalized marijuana. Also, there is limited research with respect to short-term consequences of different patterns of overlapping marijuana and alcohol use. Simultaneous alcohol and marijuana (SAM; i.e., on the same occasion so the effects overlap) use is associated with particular acute risks including serious legal, academic, interpersonal, physical and mental health problems. However, most alcohol users who use marijuana do so simultaneously. The present application proposes to conduct secondary analyses from a cutting-edge longitudinal study of YAs recruited from the community in WA State during the time of legalized marijuana. Project Transitions (R01AA022087) collected monthly data across 24 consecutive months from YAs (ages 18-25 over the course of the study). YAs (N=779) between ages 18 and 23 who reported consuming at least one alcoholic drink in the prior year were recruited and enrolled into Project Transitions. This NIAAA-funded study focused on alcohol use; however, a rich set of data on monthly marijuana use, SAM use, and related consequences is also available. The present application proposes secondary data analysis of this unique longitudinal dataset. Findings from this cost- efficient secondary analysis study utilizing monthly data will allow us to answer innovative and critically important questions that increase our knowledge of the co-occurrence of alcohol and marijuana use, whether marijuana may have substitution or complementary effects with alcohol, associations between concurrent and SAM use with short-term consequences and how these consequences may influence future use and how important naturally occurring life events (i.e., turning 21) influence trajectories of use. The project will provide answers to novel research questions that are of public health and policy importance and have important implications for young adult prevention and intervention strategies.